It is a very common practice to package food products in sealed containers to avoid contamination and to facilitate the preservation of the product. This type of packaging, widely used in the market and/or delivery of meals may also be suitable to allow the heating/cooking of the product in a thermic or microwave oven before delivering or serving the heated food while it is still inside the package. This practice is widespread in snack-bars, fast-food restaurants, canteens, vending machines for food and the like.
When these intact airtight packages are heated an overpressure is created inside the package due to the development of steam (typically water vapor) that inflates the film of hermetic package so as to cause the rupture and/or the partial detachment of welded rims, unless the film is pierced in advance. In any case, the package delivered or served after heating assumes a non-intact appearance and moreover complicates an orderly opening of a bag-type package or the removal of the film torn or partially detached from the rim of a relatively rigid container or tub.
In order to prevent these heating effects on the content of a hermetically sealed package, special valves releasing pressurized gaseous substances have been designed and developed to be pre-applied to the film which will be used to form the bag-type packaging or to seal a rigid tub or container, for releasing pressurized vapors once to a pressure threshold is exceeded, so as not to cause an uncontrolled rupture of the film from the package.
Publication JP 2006-298410 (A) describes a valve for releasing vapors which can be manufactured alignedly on a tape support. The packaging machine provides to separate a tape portion supporting a single valve, by cutting it from the tape, and to apply it in a specific area onto an inner surface of the film which it is used to form the airtight package.
U.S. Pat. No. 5,989,608, AU 2006219770 (A1), WO 2006/107974 (A1), WO 2004/108557 (A2), EP 1157942 (A1) disclose structures of adhesive label-valves applicable to the outer surface of heat-sealable films for packaging a product or preparation by sealing an opening for releasing any overpressure inside the package.
These release structures, comprising a layer of pressure-activated adhesive and a central membrane adhering to the adhesive layer to overlap the opening formed through the heat-sealable film of the package, have a relatively high cost further burdened by a high efficient waste of the used materials and a complicated application over a release opening formed in the film line of the packaging machine.
The previous patent application VA2009A000048 (Italian Patent No. 1,394,981), in the name of the same applicant, whose content is to be understood incorporated herein by reference, describes an efficient process of manufacturing flexible adhesive label-valves that minimizes any waste, simplifies the process of manufacture of the valve labels, so that they can be individually withdrawn from a feeding tape carrier by automatic manipulators that apply “alignedly” the label-valve onto the outer surface of a suitably perforated flexible film which is used to form the heat-sealed package.